Abstract : This paper addresses the problem of evaluating Human-Robot spoken interactions in a social context by considering the engagement of the human participant. We present an activity model for Human-Robot social dialogue, and show that it offers a convenient local interpretation context to assess the human participation in the activity. We describe an engagement score based on this model, and discuss results obtained by its application on a corpus of entertaining interactions involving the Nao robot in a cafeteria. We show how this approach makes it possible to discern three groups of participants in terms of engagement.